Loose gutters gutters that were improperly installed or gutters filled with debris may cause water to back up into the roof and leak through an exterior overhang.
Roof joint leak addition.
The rubber gasket that surrounds the vent pipe could be rotted.
Check plumbing vent boots and roof vents if water is entering from that area to cause the leaking roof.
Attaching below the ridge.
When a new porch is the leaking addition.
2plumbing vent boots and roof vents.
During the winter months in some.
Leak where addition meets house had similar situation as todd second roof on top of original on old section the flashing was under first layer of old roof.
Another technique for avoiding leaks involves laying rows of shingles over tar paper to prevent damage to the sheathing.
Look for cracks in the base of flexible rubber flashing or broken seams in metal varieties of vent pipe flashing.
Depending on how the roofs join you may have to cut a slot to work it in far enough.
A well ventilated attic should help prevent leaks on your roof in the winter.
When it would rain the water would run down chimney and dormers under second layer and dump into old gutters.